The present invention relates to chrome plated parts comprising substrates having industrial chrome plating applied on the surfaces thereof. The present invention also relates to a chrome plating method and a production method for obtaining such parts.
Chrome plating, especially hard chrome plating, provides a hard metallic coating (i.e., a chrome layer) having a low coefficient of friction. Therefore, chrome plating has been widely used as industrial chrome plating for parts which are required to have high wear resistance.
With respect to general-purpose hard chrome plating, a chrome layer formed on a metallic substrate contains many cracks reaching the substrate, called channel cracks. Such a chrome layer enables a corrosive material to migrate into the metallic substrate and cause corrosion. This leads to formation of red rust when the substrate is made of steel.
In producing chrome plated parts, generally, a plated substrate is subjected to polishing, such as buffing, so as to provide a smooth surface. It is known that during polishing, cracks in a chrome layer become clogged due to the occurrence of plastic flow over the surface of the chrome layer. Therefore, in producing general-purpose chrome plated parts, after polishing, no special measures have been taken to prevent rusting.
However, when a chrome layer is subject to thermal hysteresis, contraction of the chrome layer occurs. In this case, cracks which have been clogged due to plastic flow in the chrome layer are caused to open. Consequently, parts which are used at temperatures higher than room temperature (for example, at 120.degree. C. for 100 hours or more) are likely to suffer a lowering in corrosion resistance.
As a countermeasure, it has been attempted to conduct nickel plating or copper plating as a pretreatment, to thereby form a lower layer having a thickness almost equal to that of a chrome layer to be formed, and conducting hard chrome plating on the lower layer. However, in this countermeasure, a plating process must be conducted in two steps, leading to low productivity and high process costs.
As another countermeasure, it has been proposed to conduct chrome plating by using two different plating baths, to thereby deposit two chrome layers having different crystal orientations, thus preventing the formation of cracks reaching the substrate [reference is made to, for example, Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 4-350193]. However, this countermeasure also requires a two-step plating process.
Further, there is a method of conducting electro-plating with a pulse current, so-called pulse plating, so as to obtain a crack-free chrome layer [reference is made to, for example, Unexamined Japanese Patent Application Public Disclosure (Kokai) No. 3-207884]. However, the chrome layer formed simply by pulse plating is subject to tensile residual stress. This leads to the formation of large cracks in the chrome layer due to the application of heat.
Further, there is a method of conducting pulse plating in a Sargent bath by application of an irregular pulse current, to thereby obtain a crack-free decorative chrome layer [reference is made to, for example, Examined Japanese Patent Application Publication (Kokoku) No. 43-20082]. The chrome layer obtained by this method has low (or no) stress. However, the obtained chrome layer has a stress gradient (as the thickness of the chrome layer becomes large, the value of stress shifts from a side of compressive stress toward a side of tensile stress). Therefore, average compressive stress in the chrome layer is undesirably low. Consequently, when the above-mentioned chrome layer is used as a lower layer and a cracked chrome layer is formed as an upper layer by plating on the lower chrome layer, the lower chrome layer is subject to tensile stress from the upper chrome layer, so that propagation of cracks through the upper chrome layer to the lower chrome layer occurs. Further, in the chrome plating bath in Kokoku No. 43-20082, average compressive residual stress can be increased only to a level as low as 100 MPa, even by controlling the waveform of an applied pulse current, a bath temperature and a current density.
In view of the above, the present invention has been made. It is an object of the present invention to provide chrome plated parts which maintain excellent corrosion resistance even when the chrome plated parts are subject to thermal hysteresis. It is another object of the present invention to provide a chrome plating method and a production method for efficiently obtaining such chrome plated parts.